The present invention relates to aqueous phenolic resin dispersions, particularly dispersions of highly reactive, polar resole and novolak resins.
The need for waterborne compositions to replace solvent-borne compositions for purposes of reducing volatile organic compounds (VOC) is well-documented. Aqueous phenolic dispersions are known, but typically they have been prepared using protective colloids such as polyvinyl alcohol as stabilizing agents. However, polyvinyl alcohol stabilization only is effective for higher molecular weight, hydrophobic phenolic resins having a smaller number of free alkylol or hydroxy functional groups. Polyvinyl alcohol stabilization techniques are not effective for many applications requiring high polarity, highly reactive phenolic resins having a high amount of alkylol or hydroxy functional groups (see, for example, U.S. Pat. No. 5,548,015). Such highly alkylol- or hydroxy-functional phenolic resins usually have lower molecular weight and are more hydrophilic than those presently adequately stabilized by polyvinyl alcohol. If polyvinyl alcohol is tried for dispersing such highly polar and reactive hydrophilic phenolic resins, dispersion is not initially possible or the initial dispersion will phase separate over a short period of time. The polyvinyl alcohol-modified-phenolic resin molecules are polar and thus at least partially water soluble and they form larger agglomerations that are more difficult to maintain in a dispersed condition.
The difficulty of making aqueous dispersions of hydrophilic phenolic resins is addressed in U.S. Pat. No. 5,548,015 to Bourlier et al., particularly with respect to attempted stabilization with polyvinyl alcohol. According to Bourlier et al., an aqueous dispersion that includes a hydrophilic phenolic resin can be made by blending a hydrophobic etherified bisphenol A resin into the dispersion.
Water soluble phenolic resins are also known but these suffer from severe drawbacks that prevent their use to form robust, environmentally resistant films. Before being thermoset, films formed from water soluble phenolic resins tend to re-solvate when exposed to water. The source of the water can be an aqueous covercoat applied to the film. Application of the aqueous covercoat essentially washes away the film formed from the water soluble phenolic resin. In addition, water soluble phenolic resins that have been extensively ionically modified often have both low molecular weight and low reactivity thus making it difficult to obtain high crosslink density. The lack of crosslink density and the relatively high amount of ionic content means that such films exhibit very limited resistance to corrosive fluids. Moreover, water soluble novolak phenolic resins require a large amount of formaldehyde curatives. Formaldehyde has recently come under increased regulatory scrutiny.
Therefore, there is a need for an aqueous phenolic resin dispersion that includes a minimum amount of VOC (preferably is VOC-free), is storage stable for a sufficient period of time prior to use and can be easily mixed with other components to produce a useful composition. Another important desirable feature of an improved aqueous phenolic resin dispersion is superior environmental resistance of a film or coating made from the aqueous phenolic resin dispersion. In particular, it is desirable to improve the environmental resistance performance of aqueous rubber-to-metal adhesion primers that include polyvinyl alcohol-stabilized phenolic resin dispersions as described in U.S. Pat. No. 5,200,455.
The art that appears to be most closely related to the chemistry of the present invention is summarized briefly below. None of these documents, however, teach an aqueous phenolic resin dispersion.
The condensation product of 2-naphthalenesulfonic acid with formaldehyde is known as a tanning agent or a dispersing agent (see Gilbert, Sulfonation and Related Reactions, page 308 (Wiley & Sons 1965)). Along these lines, U.K. Patent Specification No. 1,469,993 relates to the use of C.sub.1-4 -alkylnaphthalenesulfonic acids or formaldehyde condensates thereof in the aqueous emulsion polymerization of chloroprene. According to an English translation, JP-B-87/047225 relates to a pressure sensitive adhesive composition that includes naphthalene sulfonic acid-formalin condensate and a desensitizing agent. The condensation product of phenol with formaldehyde in the presence of sulfuric acid is also known as a tanning agent (see U.S. Pat. No. 2,621,164).
U.S. Pat. No. 4,708,974 relates to a gel-forming composition that is derived from an aldehyde combined with various types of sulfonated phenolic compounds. The gel-forming composition is injected into subterranean reservoirs to enhance hydrocarbon recovery.
Examples 15 and 16 describe mixing a resole, aqueous formaldehyde and 2,5-dihydroxy-1,4-benzenedisulfonic acid, disodium salt and water. The resulting solution was titrated with sodium hydroxide and then heated at 100-105.degree. C. for 16 hours to produce a gel.
U.S. Pat. Nos. 4,883,824 and 4,945,077 relate to a closed cell phenolic resin foam that is made by mixing a phenolic resin with a free acid catalyst, a surface active agent and a blowing agent. Liquid frothable phenol-aldehyde resole resins are mentioned as preferred. Aromatic sulfonic acids, including phenol sulfonic acid and naphthalene sulfonic acid, are listed as preferred free acid catalysts.
U.S. Pat. Nos. 5,182,347 and 5,218,010 relate to a ceramic or refractory composition that includes magnesia aggregate, a curable liquid phenolic resin, and an accelerator for accelerating the hardening of the composition. The phenolic resin can be a resole in an aqueous solution. Naphthalenesulfonic acid and the ammonium salt of naphthalenesulfonic acid are included in the extensive list of possible accelerators.
U.S. Pat. No. 4,587,291 relates to a binder composition that includes an acid curing phenol resole resin and a curing agent that is a copper, aluminum or iron(III) salt of an aromatic sulfonic acid. Phenolsulfonic, tetrahydronaphthalenesulfonic acid, napthalenesulfonic, naphthalenedisulfonic and alkylnaphthalenesulfonic are explicitly listed as possible curing agents.
The use of sulfonated phenol-formaldehyde condensation product to impart stain resistance to polyamide materials is known (see, for example, U.S. Pat. No. 5,098,774). U.S. Pat. No. 5,098,774 relates to a method for preparing a novolak salt that includes condensing a sulfonated aromatic compound with a non-sulfonated phenolic compound and an aldehyde or aldehyde precursor then reacting the condensate with a divalent metal oxide or hydroxide or the divalent metal salt of a weak acid. Sulfonated naphthalene is listed as a possible sulfonated aromatic compound.